1. Field of the Invention
This invention relates to a semiconductor device and a method of manufacturing same.
2. Background Art
In recent years, nonvolatile semiconductor memory devices, which allow electric bulk erase and rewrite of data and in which the written data can be held without power supply, are widely used particularly in mobile devices. Such a nonvolatile semiconductor memory device is composed of memory MOS (Metal Oxide Semiconductor) transistors, which each have a tiny floating electrode (floating gate) surrounded by an insulating film, and interconnects for data input/output. The memory device retains memory by storing electric charge in the floating electrode (e.g., JP 2002-261097A).
In a nonvolatile semiconductor memory device having floating electrodes, the potential of a control electrode is controlled to control the potential of the floating electrode through the capacitive coupling between the floating electrode and the control electrode. Here, when the semiconductor substrate is grounded, the potential Vfg of the floating electrode is expressed by the following formula using the capacitance C1 between the floating electrode and the control electrode, the capacitance C2 between the floating electrode and the semiconductor substrate, and the voltage Vcg of the control electrode:Vfg=C1/(C1+C2)×Vcg where C1/(C1+C2) is called the coupling ratio.
The control gate voltage, which is needed to attain a prescribed floating gate voltage required for rewrite operation, is inversely proportional to the coupling ratio.
Nonvolatile semiconductor memory devices typically require high rewrite voltage. Here, the voltage applied to the tunnel insulating film depends on the above coupling ratio. Hence a high voltage must be applied to the control electrode. As the device structure is further downscaled in the future, a parasite capacitance α occurs in relation to the adjacent cell and decreases the coupling ratio as expressed by:Vfg=C1/(C1+C2+α)×Vcg which requires an even higher voltage to be applied to the control electrode. However, attaining the prescribed floating gate voltage required for rewrite operation simply by increasing the control gate voltage accelerates the deterioration of insulating film, and causes concern about insulation breakdown, the increase of leak current, and the decrease of reliability. To prevent this, the rewrite voltage and/or erase voltage must be reduced by increasing the charge injection efficiency and/or charge ejection efficiency of the tunnel insulating film.